OF THE COURSE BY WHICH THE BLOOD IS CARRIED FROM THE VENA CAVA
INTO THE ARTERIES, OR FROM THE RIGHT INTO THE LEFT VENTRICLE OF
THE HEART
Since the intimate connexion of the heart with the lungs, which
is apparent in the human subject, has been the probable cause of
the errors that have been committed on this point, they plainly
do amiss who, pretending to speak of the parts of animals
generally, as anatomists for the most part do, confine their
researches to the human body alone, and that when it is dead.
They obviously do not act otherwise than he who, having studied
the forms of a single commonwealth, should set about the
composition of a general system of polity; or who, having taken
cognizance of the nature of a single field, should imagine that
he had mastered the science of agriculture; or who, upon the
ground of one particular proposition, should proceed to draw
general conclusions.
Had anatomists only been as conversant with the dissection of the
lower animals as they are with that of the human body, the
matters that have hitherto kept them in a perplexity of doubt
would, in my opinion, have met them freed from every kind of
difficulty.
And first, in fishes, in which the heart consists of but a single
ventricle, being devoid of lungs, the thing is sufficiently
manifest. Here the sac, which is situated at the base of the
heart, and is the part analogous to the auricle in man, plainly
forces the blood into the heart, and the heart, in its turn,
conspicuously transmits it by a pipe or artery, or vessel
analogous to an artery; these are facts which are confirmed by
simple ocular inspection, as well as by a division of the vessel,
when the blood is seen to be projected by each pulsation of the
heart.
The same thing is also not difficult of demonstration in those
animals that have, as it were, no more than a single ventricle to
the heart, such as toads, frogs, serpents, and lizards, which
have lungs in a certain sense, as they have a voice. I have many
observations by me on the admirable structure of the lungs of
these animals, and matters appertaining, which, however, I cannot
introduce in this place. Their anatomy plainly shows us that the
blood is transferred in them from the veins to the arteries in
the same manner as in higher animals, viz., by the action of the
heart; the way, in fact, is patent, open, manifest; there is no
difficulty, no room for doubt about it; for in them the matter
stands precisely as it would in man were the septum of his heart
perforated or removed, or one ventricle made out of two; and this
being the case, I imagine that no one will doubt as to the way by
which the blood may pass from the veins into the arteries.
But as there are actually more animals which have no lungs than
there are furnished with them, and in like manner a greater
number which have only one ventricle than there are with two, it
is open to us to conclude, judging from the mass or multitude of
living creatures, that for the major part, and generally, there
is an open way by which the blood is transmitted from the veins
through the sinuses or cavities of the heart into the arteries.
I have, however, cogitating with myself, seen further, that the
same thing obtained most obviously in the embryos of those
animals that have lungs; for in the foetus the four vessels
belonging to the heart, viz., the vena cava, the pulmonary
artery, the pulmonary vein, and the great artery or aorta, are
all connected otherwise than in the adult, a fact sufficiently
known to every anatomist. The first contact and union of the vena
cava with the pulmonary veins, which occurs before the cava opens
properly into the right ventricle of the heart, or gives off the
coronary vein, a little above its escape from the liver, is by a
lateral anastomosis; this is an ample foramen, of an oval form,
communicating between the cava and the pulmonary vein, so that
the blood is free to flow in the greatest abundance by that
foramen from the vena cava into the pulmonary vein, and left
auricle, and from thence into the left ventricle. Further, in
this foramen ovale, from that part which regards the pulmonary
vein, there is a thin tough membrane, larger than the opening,
extended like an operculum or cover; this membrane in the adult
blocking up the foramen, and adhering on all sides, finally
closes it up, and almost obliterates every trace of it. In the
foetus, however, this membrane is so contrived that falling
loosely upon itself, it permits a ready access to the lungs and
heart, yielding a passage to the blood which is streaming from
the cava, and hindering the tide at the same time from flowing
back into that vein. All things, in short, permit us to believe
that in the embryo the blood must constantly pass by this foramen
from the vena cava into the pulmonary vein, and from thence into
the left auricle of the heart; and having once entered there, it
can never regurgitate.
Another union is that by the pulmonary artery, and is effected
when that vessel divides into two branches after its escape from
the right ventricle of the heart. It is as if to the two trunks
already mentioned a third were superadded, a kind of arterial
canal, carried obliquely from the pulmonary artery, to perforate
and terminate in the great artery or aorta. So that in the
dissection of the embryo, as it were, two aortas, or two roots of
the great artery, appear springing from the heart. This canal
shrinks gradually after birth, and after a time becomes withered,
and finally almost removed, like the umbilical vessels.
The arterial canal contains no membrane or valve to direct or
impede the flow of blood in this or in that direction: for at the
root of the pulmonary artery, of which the arterial canal is the
continuation in the foetus, there are three semilunar valves,
which open from within outwards, and oppose no obstacle to the
blood flowing in this direction or from the right ventricle into
the pulmonary artery and aorta; but they prevent all
regurgitation from the aorta or pulmonic vessels back upon the
right ventricle; closing with perfect accuracy, they oppose an
effectual obstacle to everything of the kind in the embryo. So
that there is also reason to believe that when the heart
contracts, the blood is regularly propelled by the canal or
passage indicated from the right ventricle into the aorta.
What is commonly said in regard to these two great
communications, to wit, that they exist for the nutrition of the
lungs, is both improbable and inconsistent; seeing that in the
adult they are closed up, abolished, and consolidated, although
the lungs, by reason of their heat and motion, must then be
presumed to require a larger supply of nourishment. The same may
be said in regard to the assertion that the heart in the embryo
does not pulsate, that it neither acts nor moves, so that nature
was forced to make these communications for the nutrition of the
lungs. This is plainly false; for simple inspection of the
incubated egg, and of embryos just taken out of the uterus, shows
that the heart moves in them precisely as in adults, and that
nature feels no such necessity. I have myself repeatedly seen
these motions, and Aristotle is likewise witness of their
reality. "The pulse," he observes, "inheres in the very
constitution of the heart, and appears from the beginning as is
learned both from the dissection of living animals and the
formation of the chick in the egg." [Footnote: Lib de Spiritu,
cap. v.] But we further observe that the passages in question are
not only pervious up to the period of birth in man, as well as in
other animals, as anatomists in general have described them, but
for several months subsequently, in some indeed for several
years, not to say for the whole course of life; as, for example,
in the goose, snipe, and various birds and many of the smaller
animals. And this circumstance it was, perhaps, that imposed upon
Botallus, who thought he had discovered a new passage for the
blood from the vena cava into the left ventricle of the heart;
and I own that when I met with the same arrangement in one of the
larger members of the mouse family, in the adult state, I was
myself at first led to something of a like conclusion.
From this it will be understood that in the human embryo, and in
the embryos of animals in which the communications are not
closed, the same thing happens, namely, that the heart by its
motion propels the blood by obvious and open passages from the
vena cava into the aorta through the cavities of both the
ventricles, the right one receiving the blood from the auricle,
and propelling it by the pulmonary artery and its continuation,
named the ductus arteriosus, into the aorta; the left, in like
manner, charged by the contraction of its auricle, which has
received its supply through the foramen ovale from the vena cava,
contracting, and projecting the blood through the root of the
aorta into the trunk of that vessel.
In embryos, consequently, whilst the lungs are yet in a state of
inaction, performing no function, subject to no motion any more
than if they had not been present, nature uses the two ventricles
of the heart as if they formed but one, for the transmission of
the blood. The condition of the embryos of those animals which
have lungs, whilst these organs are yet in abeyance and not
employed, is the same as that of those animals which have no
lungs.
So it clearly appears in the case of the foetus that the heart by
its action transfers the blood from the vena cava into the aorta,
and that by a route as obvious and open, as if in the adult the
two ventricles were made to communicate by the removal of their
septum. We therefore find that in the greater number of animals--
in all, indeed, at a certain period of their existence--the
channels for the transmission of the blood through the heart are
conspicuous. But we have to inquire why in some creatures--those,
namely, that have warm blood, and that have attained to the adult
age, man among the number--we should not conclude that the same
thing is accomplished through the substance of the lungs, which
in the embryo, and at a time when the function of these organs is
in abeyance, nature effects by the direct passages described, and
which, indeed, she seems compelled to adopt through want of a
passage by the lungs; or why it should be better (for nature
always does that which is best) that she should close up the
various open routes which she had formerly made use of in the
embryo and foetus, and still uses in all other animals. Not only
does she thereby open up no new apparent channels for the
passages of the blood, but she even shuts up those which formerly
existed.
And now the discussion is brought to this point, that they who
inquire into the ways by which the blood reaches the left
ventricle of the heart: and pulmonary veins from the vena cava,
will pursue the wisest course if they seek by dissection to
discover the causes why in the larger and more perfect animals of
mature age nature has rather chosen to make the blood percolate
the parenchyma of the lungs, than, as in other instances, chosen
a direct and obvious course--for I assume that no other path or
mode of transit can be entertained. It must be because the larger
and more perfect animals are warmer, and when adult their heat
greater--ignited, as I might say, and requiring to be damped or
mitigated, that the blood is sent through the lungs, in order
that it may be tempered by the air that is inspired, and
prevented from boiling up, and so becoming extinguished, or
something else of the sort. But to determine these matters, and
explain them satisfactorily, were to enter on a speculation in
regard to the office of the lungs and the ends for which they
exist. Upon such a subject, as well as upon what pertains to
respiration, to the necessity and use of the air, etc., as also
to the variety and diversity of organs that exist in the bodies
of animals in connexion with these matters, although I have made
a vast number of observations, I shall not speak till I can more
conveniently set them forth in a treatise apart, lest I should be
held as wandering too wide of my present purpose, which is the
use and motion of the heart, and be charged with speaking of
things beside the question, and rather complicating and quitting
than illustrating it. And now returning to my immediate subject,
I go on with what yet remains for demonstration, viz., that in
the more perfect and warmer adult animals, and man, the blood
passes from the right ventricle of the heart by the pulmonary
artery, into the lungs, and thence by the pulmonary veins into
the left auricle, and from there into the left ventricle of the
heart. And, first, I shall show that this may be so, and then I
shall prove that it is so in fact.